Gold plating solution

ABSTRACT

Provided is a cyanide-free non-electrolytic gold plating solution including a specific nitrile compound. The plating solution is sustainable and showed good bath stability and plating performance.

FIELD OF THE INVENTION

The present invention relates generally to a solution for non-electrolytic deposition of gold. More particularly, it relates to a cyanide-free non-electrolytic gold solution comprising a specific nitrile compound.

BACKGROUND OF THE INVENTION

Gold plating is used as final surface treatment for electronic industrial components such as printed circuit boards, ceramic IC packages, ITO boards and IC cards, from the standpoint of electrical conductance, solderability, physical characteristics such as connectivity by thermal crimping and oxidation resistance. Use of non-electrolytic plating rather than electroplating is preferable for most of these electronic industrial components because of the need to perform gold plating on electrically independent components and components with complex shapes.

Conventionally, due to the good bath stability and plating performance, the gold metal source in most non-electrolytic plating solutions are cyanide-based. However, owning to the toxicity of the cyanide, a sustainable cyanide-free gold plating solution is required.

Many cyanide-free gold plating solutions have been developed. Most of those cyanide-free gold plating solutions include gold sulfite compound or gold chloride compound as a gold source and a complexing agent of gold. JP03030113, U.S. Pat. No. 7,384,458 and U.S. Pat. No. 7,264,848 disclose gold plating solutions including gold sulfite compound or gold chloride compound as a gold source, and polyamino-polycarboxylic acid, disulfide compound or mercapto compound as a complexing agent of gold respectively. While these gold plating solutions are low in toxicity, the baths are either poor in bath stability at the high operation temperature (e.g. 80° C. for sulfite comprised bath) or extremely slow in deposition rate (bath with disulfide and mercapto compounds). JP2003193250A discloses a gold plating bath including gold sulfite and ammonia, with a satisfied deposition rate at low operation temperature and improved bath stability. However, the gold coating formed from the bath was non-uniform reddish brown color. JP04873196, CN102383154 and JP2005256072 disclose cyanide-free gold plating solutions including nitrogen and/or sulfur containing heterocyclic compounds. However, these gold plating solutions are either poor in bath stability (bath comprising barbituric acid and saccharin) or poor in gold adhesion on the nickel substrates (bath with hydantoin). JP61034181 discloses a gold plating solution including acetonitrile derivatives with hydroxyl groups or amino groups at the alpha position relative to a nitrile group. However, based on the study of inventors of this invention, those acetonitrile derivatives are insufficient to stabilize the cyanide-free gold plating bath. Therefore, development of a new cyanide-free gold plating solution which can deposit a uniform gold layer is desired.

SUMMARY OF THE INVENTION

Inventors of this invention have found that a gold plating solution (bath) containing cyanide free water soluble gold salt, and at least one complexing agent with the formula R—CN, wherein R is an organic radical which does not comprise a hydroxyl group or amino group in the alpha position relative to nitrile group. The nitrile complexing agent could effectively stabilize the gold ion in the solution and maintain the steady deposition rate to form a uniform gold coating. Thus, the plating performance and bath stability of the new complexing agent in a gold bath is much better than the bath only using sulfite as a complexing agent.

Therefore, one aspect of the invention relates to a cyanide free non-electrolytic gold plating solution including one or more sources of gold ions and an organic compound represented by the formula (1):

NC—R  (1)

wherein R is selected from a linear or branched, substituted or unsubstituted C₁-C₂₀ alkyl; a substituted or unsubstituted C₆-C₁₀ aryl and -A-X-A′-Y; wherein X is chosen from O, N(R′) or S, Y is chosen from —CN, amide, amino and carboxyl, A and A′ may be the same or different and are linear or branched, substituted or unsubstituted C₁-C₂₀ alkylenes and R′ is hydrogen or substituted or unsubstituted, linear or branched C₁-C₂₀ alkyl; and with the proviso that a carbon atom alpha to —CN is not substituted with hydroxyl and when the carbon alpha to —CN is substituted with an amine group, the remaining hydrogen of the alpha carbon is further substituted.

Another aspect of the invention relates to a method for forming a gold film on an article, comprising the step of contacting the article with the gold plating solution disclosed above.

Further aspect of the invention relates to an article having a gold film on its metal surface, the gold film is formed from the gold plating solution disclosed above.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout this specification, the following abbreviations shall have the following meanings unless the context clearly indicates otherwise: ° C.=degrees Centigrade; g=gram; mg=milligrams; L=liter; ml=mL=milliliters; cm=centimeters; mm=millimeters; μm=microns=micrometers. The terms “depositing” and “plating” are used interchangeably throughout this specification. The terms “non-electrolytic” and “electroless” are used interchangeably throughout this specification. All percentages are by weight, unless otherwise noted. All numerical ranges are inclusive and combinable in any order, except where it is logical that such numerical ranges are constrained to add up to 100%.

The gold plating solution of this invention includes one or more sources of gold ions. One or more gold salts which provide gold ions may be used. Since the gold plating solution is cyanide-free, the gold salts should not be alkali gold cyanide compounds such as potassium gold cyanide, sodium gold cyanide and ammonium gold cyanide. The suitable gold sources include but are not limited to, gold (I) and gold (III) halides such as gold (I) chloride and gold (III) trichloride, gold (I) bromide and gold (III) tribromide, gold (I) sulfite and gold (I) thiosulfite. The combination two or more of gold ion source can be used.

The amount of the gold ion sources in the plating solution can be generally, 0.01 to 100 g/L, preferably 0.1 to 30 g/L as gold ion.

The gold plating solution of this invention comprises an organic compound represented by the formula (1):

NC—R  (1)

In the formula (1), R is selected from a linear or branched, substituted or unsubstituted C₁-C₂₀ alkyl; a substituted or unsubstituted C₆-C₁₀ aryl and -A-X-A′-Y. Substituents of C₁-C₂₀ alkyl include but are not limited to, cyano (—CN), carboxyl, amine, amide, amino, —C(═O)NHNH₂, imide, urea, N-containing heterocycle and thiol. Examples of C₁-C₂₀ alkyl include methyl, ethyl, normal propyl, iso propyl, normal butyl, tertiary butyl, isobutyl, pentyl and hexyl. Substituents of C₁-C₂₀ aryl include but are not limited to cyano, amine, carboxyl, amide, imide and N-containing heterocycle. Examples of C₁-C₂₀ aryl include phenyl, naphthyl, phenanthryl and xenyl.

X is chosen from O, N(R′) or S. R′ is hydrogen or substituted or unsubstituted, linear or branched C₁-C₂₀ alkyl. Substituents of R′ include but are not limited to cyano, amino, amide and imide. Examples of C₁-C₂₀ alkyl include methyl, ethyl, normal propyl, iso propyl, normal butyl, tertiary butyl, isobutyl, pentyl and hexyl.

Y is chosen from cyano, amide, amino and carboxyl.

A and A′ may be the same or different and are linear or branched, substituted or unsubstituted C₁-C₂₀ alkylenes. Substituents of C₁-C₂₀ alkylene include but are not limited to oxo and azo. Examples of C₁-C₂₀ alkylene include methyl, ethyl, normal propyl, iso propyl, normal butyl, tertiary butyl, isobutyl, pentyl and hexyl.

In the formula (1), a carbon atom at alpha position to —CN is not substituted with hydroxyl because the alpha hydroxyl nitrile could release cyanide in the bath. In the formula (1), when the carbon atom at alpha position to —CN is substituted with an amine group, the remaining hydrogen of the alpha carbon is further substituted.

Preferable compounds disclosed as formula (1) include the following compounds represented by the following formulas (2) to (14).

The amount of the compound represented by formula (1) can be generally from 0.01 to 100 g/L, preferably from 0.1 to 5 g/L based on the plating solution. Such compounds may be commercially obtained or they may be made according to processes known in the art or disclosed in the literature.

The solution of the invention further includes sodium sulfite (Na₂SO₃). Sodium sulfite works as co-complexing agent in the gold plating solution. The amount of sodium sulfite in the bath can be generally from 0.02 to 200 g/L, preferably from 0.2 to 100 g/L.

The solution of the invention can further includes additives such as organic or inorganic acid, pH adjuster and/or pH buffer, such as carboxylic acid, aminocarboxylic acid, phosphoric acid, sulfuric acid, hydrochloric acid, nitric acid, sodium hydroxide, potassium hydroxide, sodium bicarbonate and sodium carbonate.

The pH of the solution is generally from 3 to 12, preferably from 5 to 10.

Another aspect of the invention is a method for forming a gold film on an article, comprising the step of contacting the article with a solution comprising one or more sources of gold ions and an organic compound represented by the formula (1). The solution used for the method is disclosed above.

Examples of the article to be plated include surfaces of printed wiring board, IC package and wafer, which have metal undercoat. Other examples are contact metal surfaces such as a terminal of wiring bonding or a connector. The metallic surfaces include copper, nickel, cobalt, silver, palladium and a metal alloy containing copper, nickel, cobalt, silver and palladium.

Time of contacting the article with the gold plating solution is basically from 1 to 120 minutes, preferably from 3 to 30 minutes.

Temperature of the gold plating solution during contacting with an article is basically from 10 to 95° C., preferably from 35 to 90° C.

Preferably, the surface of an article to be plated is formed from nickel. Nickel can be plated electrolytically or electrolessly. A preferable example when the article is nickel coated by electroless plating are the steps: cleaning an article using acid water, catalyzing the article using catalyst solution including palladium ions or colloids of palladium and tin, electroless nickel plating and gold plating.

The following examples are intended to illustrate the invention, but are not intended to limit its scope.

EXAMPLES Examples 1-4 and Comparative Examples 1 and 2

The solutions shown in Table 1 were used for examples 1 to 4. The solutions shown in Table 2 were used for comparative examples 1 and 2.

TABLE 1 Examples 1 2 3 4 Gold HAuCl₄ 1 g/L* 1 g/L* 1 g/L* 1 g/L* source Additives Na₂SO₃ 15 g/L 15 g/L 15 g/L 15 g/L Citric 20 g/L 20 g/L 20 g/L 20 g/L acid 25% ammonia 10 ml/L 10 ml/L 10 ml/L 10 ml/L Formula (1) Malononitrile Imidodiacet Cyanoacetyl Malononitrile compounds 0.66 g/L onitrile urea 0.66 g/L 0.95 g/L 1.27 g/L pH buffer Na₂CO₃/NaH Na₂CO₃/Na Na₂CO₃/NaH Na₂HPO₄/NaH₂ CO₃ HCO₃ CO₃ PO₄ pH 9.1 9.1 9.1 7.5 Plating temp. (° C.) 55 55 55 55 *Amount as gold metal

TABLE 2 Comparative Examples 1 2 Gold source HAuC1₄ 1 g/L* 1 g/L* Additives Na₂SO₃ 15 g/L 15 g/L Citric acid 20 g/L 20 g/L 25% 10 ml/L 10 ml/L ammonia Complexing agent — — pH buffer Na₂CO₃/NaHCO₃ Na₂HPO₄/NaH₂PO₄ pH 9.1 7.5 Plating temp. (° C.) 55 55 *Amount as gold metal

Process flow was shown in table 3.

TABLE 3 # Sequence Conditions Duration  1 5% RONACLEAN ™ LP-200 and 5% H₂SO₄ 40° C.   5 min  2 RO water rinse R.T. 1.5 min  3 100 g/L Na₂S₂O₈ and 5% sulfuric acid R.T.   2 min  4 RO water rinse R.T.   2 min  5 Predip: 5% sulfuric acid R.T.   1 min  6 RONAMERSE ™ SMT CATALYST CF R.T.   2 min  7 Postdip: 5% sulfuric acid R.T.   1 min  8 RO water rinse R.T.   1 min  9 DURAPOSIT ™ SMT 88 ELECTROLESS 85° C.  22 min NICKEL 10 RO water rinse R.T.   2 min 11 Immersion gold with different formulations 55° C.  10 min 12 RO water rinse R.T.   2 min 13 Dry with compressed air

A FR-4 copper clad laminates obtained from Fastprint China (test panel) was immersed in 1000 ml of acid cleaning solution (aqueous solution of 5% of RONACLEAN™ LP-200 (supplied by Dow Electronic Materials) and 5% of sulfuric acid) at 40° C. for 5 minutes. The test panel was rinsed with reverse osmosis (RO) water at room temperature (R.T.) for 1.5 minutes, and then immersed in 1000 ml of micro-etch solution comprising 100 g/L of Na₂S₂O₈ and 5% of sulfuric acid at room temperature for 2 minutes. After rinsed with RO water at room temperature for 2 minutes, the test panel was immersed in 1000 ml of 5% sulfuric acid at room temperature for 1 minute. After that, the test panel was immersed in 1000 ml of catalyst solution (RONAMERSE™ SMT CATALYST CF, supplied by Dow Electronics Materials) at room temperature for 2 minutes. Then the test panel was immersed in 5% sulfuric acid at room temperature for 1 minute. After rinsed with RO water at room temperature for 1 minute, the test panel was immersed in electroless nickel plating solution (DURAPOSIT™ SMT 88 ELECTROLESS NICKEL, supplied by Dow Electronics Materials) at 85° C. for 22 minutes. After rinsed with RO water at room temperature for 2 minutes, the test panel was immersed in the solution shown in Table 1 or 2 respectively at 55° C. for 10 minutes. Then the test panel was rinsed with RO water at room temperature for 2 minutes, and dried with compressed air.

The gold thickness of the test panel was measured by X-ray fluorescence (XDVM-T7.1-W, Ficsher) and the gold coating visual appearance was recorded. The analyzed results were shown in Table 4.

TABLE 4 Comparative Examples Examples 1 2 3 4 1 2 Au layer 0.049 0.038 0.045 0.067 0.357 0.260 thickness (μm) Au Golden Golden Golden Golden Reddish Reddish appearance yellow yellow yellow yellow brown brown

As shown in Table 4, the compound represented by the formula (1) could effectively stabilize the gold ion in the working bath and maintain the steady gold deposition rate. Thus, in examples 1-4, uniform golden yellow coatings were formed on the nickel surfaces. For Comparative Examples 1 and 2, due to the absence of nitrile compound, the gold plating rate was uncontrollable fast and a non-uniform reddish brown coating was formed on the nickel surface. 

1. A cyanide free non-electrolytic gold plating solution comprising one or more sources of gold ions and an organic compound represented by the formula (1): NC—R  (1) wherein R is selected from a linear or branched, substituted or unsubstituted C₁-C₂₀ alkyl; a substituted or unsubstituted C₆-C₁₀ aryl and -A-X-A′-Y; wherein X is chosen from O, N(R′) or S, Y is chosen from —CN, amide, amino and carboxyl, A and A′ may be the same or different and are linear or branched, substituted or unsubstituted C₁-C₂₀ alkylenes and R′ is hydrogen or substituted or unsubstituted, linear or branched C₁-C₂₀ alkyl; and with the proviso that a carbon atom alpha to —CN is not substituted with hydroxyl and when the carbon alpha to —CN is substituted with an amine group, the remaining hydrogen of the alpha carbon is further substituted.
 2. The gold plating solution of claim 1, wherein the organic compound is selected from the compounds represented by the formula (2) to (14),

wherein n is a integer from 1 to
 10. 3. The gold plating solution of claim 1, further comprising sodium sulfite.
 4. The gold plating solution of claim 1, wherein the gold source is selected from gold chloride, gold sulfite and gold thiosulfite.
 5. The gold plating solution of claim 1, wherein the concentration of the organic compound represented by the formula (1) is from 0.1 to 5 g/L.
 6. A method for forming a gold film on an article according to claim 1, comprising the step of contacting the article with a solution comprising one or more sources of gold ions and an organic compound represented by the formula (1).
 7. An article having a gold film on its metal surface, wherein the gold film is formed from the gold plating solutions of claim
 1. 8. The article of claim 7, wherein the metal surface is selected from the group consisting of copper, nickel, cobalt, silver, palladium and a metal alloy containing copper, nickel, cobalt, silver and palladium. 